To help understand new concepts, I like to simplify them with easy to remember relationships. For the two-wire 4- 20mA current loop, I use a simple triangle as follows:
From the triangle, I can quickly identify the three common components of a current loop, how they are wired together,and even the direction of current flow. For my mental model, each side of the triangle represents a component of the current loop. The vertices of the triangle represent a wired connection between these components. For reference, I also place a positive/plus sign on the “peak” of the triangle. I will also use the first three letters of “Triangle” to identify the principle components. Accepting the convention that current flow will move from the supply positive to and return to the supply negative, I see that current moves counter-clockwise in my symbolic current loop.
The left side is labeled “T” and refers to “Transmitter”. The base of the triangle is labeled “R” and refers to “Receiver”. The right side of the triangle is labeled “I” and refers to the source of current or the power supply. The vertices refer to the wired connections between these elements. Each element shares two connections to the loop. The “+” label at the peak shows that the positive side of the power supply connects to the positive side of the transmitter. Using the convention of current flow from positive to negative, the direction of current flow from the power supply is shown counter-clockwise. You should also note that the Transmitter is not the source of current, but simply regulates the flow and magnitude of the current through it. The current is sourced by the power supply, flows in controlled fashion through the transmitter, then into the receiver, and returns to the power supply. The current flowing through the receiver produces a voltage that is easily measured by the analog input of a controller or monitoring device. Figure 2B shows a schematic of a simple 4-20mA current loop. Note the traditional position of earth ground in the current loop of Figure 2B.
2 wire transmitter system is given in below:
Some transmitters need more power than can be supplied over the 4-20mA loop for their operation. In this case, a separate power supply will be needed, requiring the extra 2 wires. Otherwise, the instrument derives its power from the loop.
Typical examples include:
“It's not a question as to which is better, 2-wire or 4-wire”. A magnetic flowmeter could not be used on low conductivity fluids or steam, for example; a vortex-shedding flowmeter would be affected by solids in the fluid whereas a mag-flow is not. Selection takes place on the basis of the suitability of the measuring technique, the size, its availability for use in a zoned hazardous area (many 4-wire instruments are not suited) and, partly, the cost. 2-wire or 4-wire is rarely a consideration in the selection process.
Look at this video , I had explained and showed that how 2,3& 4 wire connection configuration work ,difference and explained how the signal and power flow. i am attaching a snap of the video.
here i the link.
Have a happy learning
Hello,
In a 2 wire transmitter, the device is powered by the same 2 wires that carry the 4–20 mA signal. Ideally this instrument should consume less than 4 mA to operate, in order to enable the device to function in the full 4–20 mA range.
Sometimes, when the devices need more than 4mA for their own operation, the instrumentation makers make their devices available only 8–20 mA or 12–20 mA.
If you need full 4–20 mA range and/or if the device you’ll be using consumes more power, then you’ll have to go to the 4 wire transmitter option.
In a 4 wire transmitter the device is powered by a pair of wires (24 VDC typically) and the 4–20 mA signal goes in another pair.
The two wires are mostly used because you save money in cable as the copper wires are expensive.
Hope to have been of your assistance but, nevertheless, I do remain at your entire disposal in the meantime.
Kindest regards from Portugal / David
1 - Port Y is the balancing impedance
2 - Port W is the telephone line
3 - Ports X or Z are interchangeable as the transmit output or the receiver input.
In instrumentation, a 2-wire transmitter carries both the signal from the gauge and the power to the gauge on a single pair of wires. This may either be a 4-20 mA type arrangement, or it may be DC power and a strictly AC signal (isolated with capacitors or transformers), or the gauge may not need power (as in a piezoelectric transducer) but a pair of signal wires, only.
In a 4-wire system, the power to the gauge is on one pair of wires, while the signal is carried on the other pair of wires. Bridge-type gauges usually use a 4-wire system.
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